The line comes from the 1980 comedy Airplane!, written and directed by Jim Abrahams, David Zucker and Jerry Zucker. Leslie Nielsen plays Dr Rumack, a straight-faced doctor on a flight in crisis, and the Shirley joke is one of the film's most quoted exchanges. It spread through television repeats, home video and later meme culture, where the deadpan delivery made it perfect for reaction clips and short edits. Decades later it still works because the pun is simple, the setup is familiar, and the humour comes from saying something absurd with total seriousness.